i have no idea what this is. The rear brake line goes to it, then back to the other side and on to the diff.

Ok, Nissan part type 46400 is:
46: Master cylinder, pedal &, piping group
400: Load Sensing Valve Assembly

i was wrong again...

So I shouldn't remove that, I'm assuming? I may bypass it tonight anyway, just to see. There's not enough wheel travel on my truck for that thing to work in the first place.

It's check valve to maintain brake shoe position. With discs now fitted it should be removed and bypassed.

That's in the master cylinder.

The load sensing valve normally has a lever on it, to adust brake proportioning.

So I removed the valve, and no change, so that's not it. I probably goofed rebuilding them somehow... I'll report back when I figure it out.

hey josh does that valve move somehow?

There is an angled channel in the sheet metal right there, I wonder if that moves the valve somehow when there is heavy load in the bed...?

No, it was solid mounted. Looks like there's room inside it for something to slide back and forth. I got some calipers from a friend and swapped them at lunch today. I must have buggered something on the rebuild because they work great. Now I just need to work out the e-brake.